This invention relates to optical switches.
An optical switch is used in an optical time domain reflectometer (OTDR) to control propagation of light from a light source within the reflectometer, typically a laser diode, to a fiber under test and from the fiber under test to a detector, such as an avalanche photodiode. As shown in FIG. 1, a switch that may be used for this purpose comprises a substrate 2 of Z-cut LiNbO.sub.3 having diffused titanium waveguides 4-8 formed therein and electrodes 12, 14 deposited on the substrate. The waveguide 6 has two segments that extend adjacent respective segments of waveguides 4, 8 so that two directional couplers 16A, 16B are formed. The waveguide segments that form the directional couplers are positioned so that the Z component E.sub.z of the electric field established by electrodes 12, 14 interacts differentially with the optical modes propagating in the waveguide segments. Thus, a voltage applied to electrodes 12, 14 by an electrode driver 18 establishes an electric field that can affect the coupling of light between the waveguides.
Electrode driver 18 is able to establish two distinct states for the optical switch In a first state of the switch, the directional couplers are each in the bar state, and light from the laser diode 20 is coupled through a fiber 21, waveguide 4 and coupler 16B into the fiber under test 22. In the first state, reflected light entering waveguide 4 from fiber 22 is coupled into fiber 21. In the second state of the switch, couplers 16A, 16B are each in the cross state and light from the fiber under test is coupled to detector 24 through waveguide 6, couplers 16A, 16B and waveguide 8.
The fraction of light that is coupled across a directional coupler is known as the coupling ratio of the coupler. The switch transmission of a switch in a given state is the fraction whose denominator is the optical power entering the switch at one fiber port and whose numerator is the optical power emitted by way of another fiber port in that state, and is a combination of the coupling loss (the complement of the coupling ratio) for each coupler and the non-coupling losses (waveguide losses and fiber/substrate interface losses), which are typically about 3 dB.
In operation of the OTDR, a laser driver (not shown) energizes laser diode 20 to emit a brief light pulse, and electrode driver 18 places the switch in the first state, so that the light pulse is coupled into fiber 22. For a short time following launch of the pulse into fiber 22, reflected light is received from fiber 22. During intervals in which the reflections are weak, the switch is placed in the second state and the reflections are coupled to detector 24. During intervals in which strong reflections are received, the switch is placed in its first state, and detector 24 is optically isolated from fiber 22. The manner in which the electrode driver is controlled to place the switch in the first or second state when needed is well understood by those skilled in the art. Suitable techniques are described in co-pending U.S. patent application Ser. No. 07/329,796 filed Mar. 28, 1989.
The reflected light received from fiber 22 may be polarized. In order that the polarization state of reflected light should not influence the detection of the intensity of the reflected light, it is essential that the optical switch should pass all polarizations equally, i.e., the switch should be polarization independent. In the case of a switch of the kind shown in FIG. 1, this necessitates that the switch transmission of the switch in the cross state be the same for the TM and TE modes.
In Y. Tanisawa, T. Aoyama, R. Ishikawa, M. Kondo and Y. Ohta, "Low Crosstalk and Polarization Independent Ti:LiNbO.sub.3 Optical Waveguide Switch for OTDR", Proceedings, Conference on Integrated and Guided Wave Optics, 240 (1988), polarization independence of the passive coupling ratios of a switch of the type shown in FIG. 1 is achieved by controlling both the concentration with which Ti is diffused into the LiNbO.sub.3 to form the waveguides and the spacing between the waveguides. However, the waveguide losses and fiber/substrate interface losses are polarization dependent, and therefore the switch transmission of a switch of the kind shown by Tanisawa et al will not be polarization independent.
For light of a given wavelength and polarization, the passive coupling ratio of a directional coupler is a function of the interaction length of the coupler (a physical characteristic of the coupler). The function is periodic, and the period of the function is called the coupling length. The coupling length is a function of wavelength and polarization. For a given wavelength and polarization, the passive coupling ratio is a maximum when the interaction length is equal to an odd multiple of the coupling length for that wavelength and polarization and is a minimum when the interaction length is equal to an even multiple of the coupling length.
The coupling length for a given wavelength and polarization is an exponential function of the separation between the waveguides. The coefficients of the exponential relationships for the two polarizations (TM and TE) at a given wavelength are not the same and therefore the TM and TE coupling lengths can be varied with a degree of independence. Accordingly, a directional coupler can be fabricated so that the interaction length is equal to both an odd multiple of the TM coupling length and an odd multiple of the TE coupling length. Such an interaction length is referred to herein as an "optimum interaction length". L. McCaughan, "Low-Loss Polarization-Independent Electrooptical Switches at .lambda.=1.3 .mu.m", IEEE J. Lightwave Tech., LT-2, 51 (1984) describes a switch based on a directional coupler in which the number of coupling lengths for the TE and TM polarizations are equal and odd numbered.
Although the passive coupling ratio of a directional coupler in which the interaction length is equal to both an odd multiple of the TM coupling length and an odd multiple of the TE coupling length is polarization independent, polarization-dependent effects occur where the fibers are coupled to the substrate and in propagation through the substrate. Consequently, when a directional coupler having an optimum interaction length is used in the optical switch of an OTDR, propagation of light from the fiber under test to the detector in the cross state is not polarization independent.
FIG. 2 shows the coupling ratio of a directional coupler as a function of electrode voltage for a directional coupler for which the interaction length is equal to an optimum interaction length. Curve A relates to the TM polarization and curve B to the TE polarization. As shown, in the cross state, i.e. with a zero electrode voltage, 100% coupling takes place from one waveguide to the other. For a non-zero electrode voltage, the coupling ratio is reduced. Curves A and B in FIG. 3 show the switch transmission for the TM and TE polarizations from the fiber under test to the detector of an OTDR by a switch based on a directional coupler whose coupling ratio varies with voltage in the manner shown in FIG. 2. As shown in FIG. 3, at zero electrode volts, both the TE and TM polarizations suffer losses, and these losses are unequal. Specifically, the TM polarization suffers greater loss than the TE polarization. Consequently, the coupling from the fiber under test to the detector is not polarization independent.
The TM and TE coupling lengths are wavelength dependent. Therefore, a directional coupler having a polarization-independent coupling mechanism provides 100% transmission in the cross state only for a single wavelength. Curves A and B in FIG. 4 show passive coupling ratio for the TM and TE polarizations as a function of wavelength for a directional coupler having an interaction length equal to the TM coupling length for light at 1300 nm and to three times the TE coupling length at that wavelength. It will be noted that the two curves are similar except that the roll-off with wavelength is not as severe for the TM polarization as for the TE polarization. It can be seen that for the TE polarization the passive coupling ratio decreases quite rapidly as a function of wavelength and may be only about 80% at a wavelength less than about 1295 nm or more than about 1305 nm. Therefore, if such a directional coupler is used in the optical switch of an OTDR, the performance of the switch, and thus the sensitivity of the OTDR, depends strongly on the wavelength of the laser diode. Moreover, if two directional couplers are connected in series, as shown in FIG. 1, the roll-off is even more rapid. The processes for fabrication of laser diodes are not sufficiently developed that a wavelength difference of less than about 10 nm can be achieved from diode to diode in a production run. Consequently, if the laser diode in an OTDR fails and is replaced, the sensitivity of the OTDR after replacement is generally significantly different from its sensitivity before replacement unless the diodes have been matched with respect to wavelength.
In Alferness, U.S. Pat. No. 4,243,295 issued Jan. 6, 1981, the concept of spatial tapering (varying the distance between the waveguides of a directional coupler as a function of length within the domain of the interaction length) is described. This spatial tapering makes it possible to achieve high isolation in the bar state for both polarization states at a common drive voltage.